fix rm dir rule to make it case insensitive

In bash the output for the command `rm -f foo/` is:

    rm: cannot remove ‘foo/’: Is a directory

And not:

    rm: cannot remove ‘foo/’: is a directory

thefuck/rules/rm_dir.py

@@ -5,7 +5,7 @@ from thefuck.utils import sudo_support
 @sudo_support
 def match(command, settings):
     return ('rm' in command.script
-            and 'is a directory' in command.stderr)
+            and 'is a directory' in command.stderr.lower())
 
 
 @sudo_support